basically my amp was having volume issues and most people recommended I swap out the power tubes. I swapped out the preamp tubes after also replacing the power tubes and my problem is still unsolved. There is an interesting thing though :

After installing the new tubes I was pretty frustrated so I just left my amp alone for a couple weeks. Finally I turned it back on and voila! It was as if nothing had ever gone wrong. I was able to play at a low volume all week until practice came around. Well during practice I manage to get through one song at full volume and in the middle of the second song my volume drops to about 10%. All I got was the fizzy quiet tone described in my original post. Does anybody have any ideas as to what it might be? Could it be a faulty input jack? Thanks in advance for any replies.

I assume you tried different cables and guitar? And took all pedals out of the chain to insure that isn't the problem?

Assuming that, try cleaning the jacks with some contact cleaner. Spray a little on a plug and work it in and out. I'd do the same with the tube sockets. Spray a little on the pins of the tubes and work them in and out of the socket.

If you're still having problems, replace the input jack and the speaker jack (assuming it has one) with some switch crafts and see if that doesn't solve the problem.

I've done everything you suggested Kevin. I'm not sure what you mean by switch crafts. Is it difficult to switch out the jacks? I don't have a ton of experience with a soldering iron, but I'm not terrible with it. I just have shaky hands.

@R45VT I've used both the cable from my combo to my cab as well as the cable from my amp to the speaker that it came with. Doesn't seem to make a difference. It sounds like you guys think it could be faulty jacks somewhere huh?

When it happens it stays that way. Before It appeared that I could play indefinitely at a low bedroom volume without any problems. It didn't last more than 7-10 minutes at a full practice volume.

I just turned it on after a couple of weeks without using it and it looks like the time off didn't help like last time. The main symptom is just really low volume. The tone isn't terribly degraded, but even at max volume, I couldn't play with a drummer. The only thing is that the tone is a little farty if there is a lot of low end. It just sounds like a weak signal. Yes it does it on both channels.

To answer Cathbard's question, there isn't any extra noise associated with the drop in volume. Before this all happened though, I noticed there was some crackling going on while i was playing, especially if there was a spike in the higher frequencies. I just chalked this up to old tubes though.

I would make a recording, but I don't think you'll notice anything unusual. The tone sounds pretty normal, without any extra noise. It's just about 200 - 300% quieter.

I think I've answered all the questions from last night. Hope you guys see this.